Dried paint often comes into contact with itself, for example, in window and door areas. Depending on the dried paint's hardness, the pressure, the temperature, the humidity, and the duration of time in which the surfaces are in contact, the painted surfaces sometimes stick together. This undesirable sticking together of two painted surfaces when pressed together or placed in contact with each other is referred to as “blocking.” Thus, an important characteristic of coatings is the block resistance.
The glass transition temperature (Tg) of a polymer is an inherent physical property of the monomer or monomers used to make a polymer included in a coating composition. The Tg of a polymer determines the relative hardness or softness of the polymer. The higher the polymer's Tg, the harder the polymer, and the lower the polymer's Tg, the softer the polymer. As such, the Tg of a polymer can help to determine the physical characteristics of a film formed from a coating composition containing the polymer. The Tg of the polymer can also help to determine the minimum temperature at which the coating composition containing the polymer can be applied to a substrate to form a film, or the minimum film forming temperature (MFFT). The MFFT is the lowest temperature at which the polymer particles of the coating composition will mutually coalesce and form a continuous film when the water evaporates.
In order to provide a coating with the ability to form a film hard enough to avoid tackiness, blocking, and dirt pickup, polymers with blends of polymers having different Tg values have been used in coating compositions. By increasing the Tg of a polymer useful as a binder in a coating, the hardness of the final coating also increases. This is useful since the hardness of a coating affects other desirable properties of the coating, such as, for example, block resistance. However, increasing the Tg of a polymer can also create a coating with a high MFFT.
Coalescing solvents normally are required in coating compositions since it is desired that the coating composition has the lowest possible MFFT and the highest possible glass transition temperature. Coalescing solvents are organic solvents or plasticizers that effectively lower the MFFT of the polymer to meet the desired low MFFT on application, and then eventually diffuse out of the coating composition and evaporate under normal ambient conditions of temperature, humidity, and atmospheric pressure, leaving a high Tg film.
Although the use of coalescents has proven to be a very useful way to solve the problem of obtaining certain desired film properties with high Tg polymers, which do not readily form films at desired application temperatures, this solution has created another problem. During the drying of a coalescent containing formulation, the organic solvents evaporate and enter into the atmosphere. In addition to the unpleasant odor associated with these organic solvents, there is growing concern about the potentially adverse environmental and health effects of many of these organic solvents.
As such, there is a growing need for polymers for use in coating compositions, which will provide desired hardness properties, adequate film formation at low temperature, and flexibility. In addition, it is also desirable to eliminate volatile coalescents without compromising physical properties such as coating hardness and low MFFT.